As a result of the advance of radio communication technologies, radio communication systems such as mobile phone systems are currently in wide use. When a communication device in a radio communication system transmits a radio signal, a digital signal to be transmitted is converted to an analog signal, then modulated and amplified by a transceiver amplifier. The analog signal generated in this manner is subjected to band limiting in a band pass filter (BPF) and then output from a transceiver antenna. Radio resources are limited, and thus, there has been a demand for further improvement in signal transmission efficiency to cope with the growth of radio communication systems.
One cause of the reduction in the transmission efficiency is amplification distortion caused in the amplifier. The amplifier is expected to have a linear relationship between the input and output signal levels. However, because of its physical properties, the amplifier exhibits nonlinearity in a range of the input signal level exceeding a predetermined level. Such nonlinearity of the amplifier brings about amplification distortion of the signal, causing reduction in the transmission efficiency. To cope with this, distortion compensation techniques have been proposed wherein, in a transceiver amplifier, a digital signal is previously applied with a distortion opposite to the amplification distortion caused in the amplifier, to suppress the amplification distortion of the analog signal having passed through the amplifier. In many distortion compensation techniques, a feedback circuit is used to convert the analog signal that has passed through the amplifier to a digital signal to detect distortion (see, e.g., Japanese Laid-open Patent Publications Nos. 2007-53552 and 11-284675).
Another cause of the reduction in the transmission efficiency is delay deviation caused in the band-pass filter. In the band-pass filter, delay attributable to the filtering process varies depending on the frequency. Namely, there arises a time difference in output timing between high- and low-frequency components of the input signal. Such delay deviation caused by the band-pass filter results in an increase in the signal decoding error at a receiving-side device, causing lowering of the transmission efficiency. To cope with this, delay deviation compensation techniques have been proposed wherein, in a transceiver amplifier, a digital signal is previously given a delay for canceling out the delay deviation caused by the band-pass filter, to suppress the delay deviation between the frequency components of the analog signal having passed through the band-pass filter (see, e.g., Japanese Laid-open Patent Publication No. 07-202955).
In recent years, removal of unwanted signals outside the transmission frequency band has been strictly demanded, with the result that the role of the band-pass filter has become more and more important. For example, in some applications, the desired signal power-to-interference signal power ratio at a frequency spaced by 5 MHz (megahertz) from the transmission frequency band is desired to be 110 dB (decibels) or higher.
Generally, a band-pass filter capable of more effective attenuation of unwanted signals outside the transmission frequency band tends to have a greater delay deviation between the frequency components. For this reason, there has been a demand for a delay deviation compensation technique capable of appropriately suppressing the occurrence of decoding error at the receiving-side device. With the technique disclosed in Japanese Laid-open Patent Publication No. 07-202955, however, it is difficult to set an optimum correction amount, in terms of transmission efficiency, for the delay deviation compensation. Consequently, the occurrence of decoding error at the receiving-side device fails to be satisfactorily suppressed.